Mrs. Mvubelo, a black trade unionist, was one of a growing number of victims whose homes have been attacked with petrol bombs by those who have adopted a violent hard-line stance against people they regard as government ''stooges.''

Mrs. Mvubelo, general secretary of the National Union of Clothing Workers of South Africa, has been criticized by some blacks because of her call for continued foreign investment in this country.

Frequently targeted these days are political figures who are participating in institutions created by South Africa's ruling whites. Most of the attacks appear to be occurring in Soweto.

A group calling itself the South African Suicide Squad (SASS) has claimed responsibility for many of the incidents. It is the first time many here have heard of the group, and the police admit to knowing little about the organization or its roots.

The main black nationalist group embarked on a sabotage campaign in South Africa is the outlawed African National Congress. And while the ANC is believed responsible for past attacks on certain individuals - black policemen and black informers - there is as yet no clear link between the ANC and the new wave of bombings.

Over the past week at least a half dozen homes have been hit with bombs - usually a rag stuffed into a bottle filled with gasoline - that have done structural damage but have so far not seriously injured any of the political figures targeted. In the past, though, these types of attacks have claimed lives.

In addition to black local councilors in Soweto, those under attack include Coloreds (persons of mixed-race descent) and Indians who are participating in the elections scheduled for next month. These elections are for Colored and Indian representation in a new-styled parliament that is rejected by many government critics. The argument against the parliament is that it excludes blacks.

Black councilors are under attack because many blacks reject the system of local government in which they operate. Criticism of the black local councils say that even in black hands, they can do nothing but perpetuate apartheid. Only about 11 percent of blacks voted in local council elections last year.

The Soweto city council announced earlier this year that it would install a protection system to ensure the safety of its members from bombings.

The South African police have not yet been noticeably effective in thwarting this new wave of violence. Some analysts believe South Africa can expect to see more and more of this kind of activity as the government continues to try and bring more and more ''nonwhites'' into ''the system.''

Opposing ''collaborators'' is ''not a new principle'' in the battle against South Africa's white government, says Michael Hough, director of the Institute for Strategic Studies at the University of Pretoria.

But the use of violence against collaborators - who used to be the subject of mainly verbal assault - appears to be growing, Mr. Hough adds. And he says the trend could intensify ''as more and more people (nonwhites) are being drawn into the government system.''